The present invention relates generally to catalytic reaction systems, and more particularly to microlith catalyst beds for improving catalytic conversion efficiency.
In the past, typical catalytic convertors for oxidation of carbonaceous fuels, for example as used in automotive emissions, have used unitary structures such as an assembly of interlocking sheets of catalyst material to form a catalyst bed. This type of structure is referred to as a monolith catalyst. However, the performance of monolith catalysts have been limited by the fact they are not generally effective until the catalyst has heated up to its operating temperature. Monolith catalysts suffer from long warmup (lightoff) time. Further, they generally do not provide satisfactory high catalytic conversion efficiency.
In U.S. Pat. No. 5,051,241, William C. Pfefferle, discloses a microlith catalytic reaction system which provides a more effective catalytic conversion than conventional monolith converters when operating in what is known as the mass transfer limited region of catalyst operation. In the mass transfer limited region of operation, the reacting chemical species must diffuse through a boundary layer and reach the catalyst surface for the catalyst to be effective in its function of accelerating reaction rates. The microlith catalyst system of Pfefferle provides quicker lightoff and higher conversion efficiency due to high open area microlith catalyst elements having flow channels with a flow path length no longer than about two times the diameter of the largest flow channel. Pfefferle further teaches the ability to electrically heat the microlith catalyst to further reduce the lightoff time.
The system of Pfefferle utilizes multiple layers of microlith catalyst support to obtain the total surface area required to achieve the desired reaction rate. To provide the electrical heating, an electrical path can be implemented by either a series connection passing through each layer of microlith catalyst with jumper connections between each layer, or a parallel connection where a common power bus connects all layers to the power supply. Both of these wiring configurations require numerous electrical connections that drive up cost and reduce reliability of the system.
It is also noted that in many applications, the microlith layers will be circular disks cut from flat sheets of fine mesh screen or expanded metal. This operation may result in a significant quantity of leftover scrap. Further, if the catalyst material is applied to the fine mesh at the time the mesh is manufactured, the cost of the leftover scrap could be very expensive.
Further problems occur with prior art catalyst support designs when applied to catalytic combustors used in gas turbine engines. Typical catalytic combustors operate in what is known as the homogenous gas phase reaction region of catalyst operation. In this region, most of the chemical reactions take place in the freestream off of the catalyst surface. The catalyst contributes to the overall reaction by reacting with a small portion of the total reactant stream to promote the formation of chemical radicals. These radicals in turn increase the reaction rate of the chemical species reacting in the freestream.
However, when gas turbines are operated at low power outputs, the catalytic combustor may be forced to operate in the slower mass transfer limited range (described hereinabove) due to cooler operating temperatures. Since gas turbine catalytic combustors are typically designed for minimum volume, the combustor may not have sufficient catalyst surface area to operate efficiently in the mass transfer limited region.